MAGNETIC FIELD DYNAMICS IN PLASMA OPENING SWITCH

Summary form only given, as follows. Plasma opening switches (POS) are the key components of inductive energy store (IES) pulsed power systems. In the closed phase an IES system stores magnetic energy in the inductance of a vacuum transmission line. As the POS opens the stored magnetic energy flows past the POS and into a load (for example an electron or ion diode). To achieve power multiplication the opening time should be much shorter than the conduction time. There are three proposed mechanisms that control the conduction phase of the POS: 1) sheath effects at the electrodes, 2) rapid penetration of magnetic field into the body of the plasma due to EMH effects, and 3) displacement and distortion of the plasma. due to J/spl times/B (MHD) forces. In this paper, the PIC code, MAGIC, is used to study EMH and MHD effects in a POS. These simulations were performed in negative polarity with a strong radial density gradient. In regimes where EMH is expected to be dominant, the simulations show rapid axial penetration of magnetic field into the plasma in a narrow radial channel along the positive density gradient. No field penetration is observed along the negative portion of the density gradient. In positive polarity, field penetration occurs along the negative gradient and no field penetration is observed along the positive density gradient. This agrees with EMH theory of shock penetration of magnetic field. However, the speed of penetration is somewhat slower than predicted by fluid treatments. The PIC simulations also show the formation of vortices in the electron flow behind the EMH shock front.

Summary form only given. Conduction and opening processes of plasma opening switches (POS) are examined with the aid of numerical simulations. In a POS, a plasma initially bridges a small axial region of the transmission line of an inductive energy storage generator. A section of vacuum transmission line then connects the POS region to a load. The plasma initially carries the generator current, allowing no current to flow to the load. Modifications to the plasma or to the current distribution in the plasma eventually open the POS, allowing power to flow to the load. These modifications can occur by a variety of different mechanisms: by plasma deformation and displacement, by magnetic field transport, or by a combination of such effects. Deformation and displacement are carried out by J/spl times/B forces and by electrostatic forces (ion erosion) associated with electron vortices. Magnetic field penetration can occur by electron-magneto-hydrodynamic (EMH) effects or resistive diffusion. Both particle-in-cell (PIC) and MHD codes are used to demonstrate many of these effects. In these simulations, a wide range of plasma densities (n/sub e/=5/spl times/10/sup 12/-5/spl times/10/sup 15/ cm/sup -3/) are examined.

A coaxial geometry plasma opening switch (POS) has been tested on the 4 MJ ACE 4 pulsed power driver. Simple models of POS operation were used to scale successful results from drivers such as HAWK at NRL and ACE 1 which operate at the desired conduction time but at currents below 1 MA. For the conduction phase, a snowplow model indicates that the square root of the plasma density is proportional to the product of conduction time and peak conduction current for fixed radius and length switches. This relationship was verified for the ACE 4 POS. For the opening phase, a model of gap formation enabled plasma density thinning during the conduction phase was applied. The gap distance can be estimated from the voltage and current measurements using a magnetic insulation model.

The article is dedicated to optimizing the digital current control loop of a thyristor converter-driven electric drive, essential for ensuring stable, precise, and reliable operation of electric drives in automated and industrial systems. The primary goal of this research is to develop and enhance the digital current regulator (DCR), which enables the drive system to achieve desired performance, including rapid and stable response to current changes without excessive overshoot or oscillations. Optimization of the current loop improves energy efficiency, enhances dynamic properties, and increases resistance to external disturbances, all of which are critical for modern electric drive systems. The introduction discusses key aspects of digital current loop optimization, specifically dynamic characteristics such as time delays, sampling rate, and bandwidth, which significantly impact the efficiency of digital controllers. Special attention is given to the tuning of proportional-integral (PI) controller parameters, allowing quick and stable response to current changes. The developed model allows preliminary simulation of the current loop’s performance, providing an opportunity to test different configurations before actual implementation. To improve the accuracy and stability of the system, filtering methods are proposed to reduce the impact of high-frequency noise, ensuring the system’s precision and robustness against external interference. Optimization criteria are introduced, including minimizing the integral square error (ISE) and reducing energy consumption, which significantly decreases system losses and increases the energy efficiency of electric drives. In developing the optimized control loop, the following assumptions are considered: 1) the thyristor converter (TC) operates in continuous current mode, 2) the current loop dynamics are studied within a narrow range of TC phase angle changes, and 3) a discrete dynamic TC model is used with averaging of electromotive force (EMF) over the conduction interval. Optimizing the digital current loop addresses several critical tasks, including enhancing control accuracy, reducing overshoot and oscillations, and improving the system’s dynamic response, allowing for reduced delays and a faster reaction to load changes. The optimization results of the digital current regulator (DCR) show reduced energy consumption and significant improvements in control accuracy. The high-speed performance of the digital current loop enables fast transient processes within a single sampling interval of the thyristor converter, which is crucial for systems requiring rapid regulation. Additionally, the digital control loop provides flexibility in control, as it allows for system adjustments to different operating conditions through a software-based approach. The study’s final results demonstrate the potential of digital current loops in modern electric drives. With advances in microprocessor technology, digital regulators enable more complex control algorithms, including adaptive and predictive strategies, which further enhance system efficiency. These approaches are especially relevant for industrial electric drives that require high precision and response speed, making digital current loops a promising solution for contemporary automated control systems.

Experiments with the microsecond plasma opening switch were carried out on the GIT-12 installation at the conduction current level of 3 MA. In this series an attempt to increase both the critical charge responsible for the conduction phase and the POS voltage was undertaken. The necessity of such improvement is conditioned by the difficulties of application of existing POS technology for construction of a new, multi-MA class IES driver. The experiments were inspired by several phenomenological ideas and theoretical results described as the main background philosophy of this work. Possible improvement of the POS operation is predicted if using the following technique: 1) optimization of the plasma injection length in order to increase the critical charge by keeping the initial plasma density at minimum possible level; 2) use of the initial plasma density gradient in the direction from the load to the generator in order to decouple the plasma densities responsible for the conduction and opening times; 3) use of the controlled plasma motion during the conduction phase for abrupt enhancement of the magnetic field prior to the opening phase. A composite POS satisfying these conditions and consisting of initially prepared MHD and Hall parts was applied. The first, mainly proof-of-principle results from C-POS operation on GIT-12 are reported.

Summary form only given. Ferroelectric plasma sources are known as sources of surface discharge plasma. Main advantage these sources is the ability to create layers of plasma on surfaces with large area and arbitrary shape. In vacuum, numerous discharges on the surface of ferroelectric ceramics appear in response of a driving pulse applied across a sample of ferroelectric ceramics. Electron avalanches can be initiated by field emission from triple junctions at the edges of patterned electrodes in electric fields, induced by inhomogeneities of the polarization charge. These avalanches along the surface are driven by secondary electron emission form the ferroelectric ceramics. Surface ablation and gas desorption supply neutrals, which are further ionized by avalanching electrons, resulting in a surface plasma. The depicted scenario is the same as for surface flashover; however, discharge on ferroelectrics does not turn to vacuum arc because of separation of electrodes by the ceramic bulk. This feature makes the discharge on ferroelectrics very interesting for study of the plasma formation process in the surface discharge. For instance, initial plasma density and electron temperature in the stem of the discharge can be deduced from the temporal profiles of the density in expanding plasma. For PZT ceramics, the initial plasma density was found in the range of (0.4-4)/spl times/10/sup 17/ cm/sup -3/ and the initial electron temperature of about 16-20 eV for the driving electric fields of 16-30 kV/cm. It is shown that the discharge on ferroelectrics can be realized also in a gas mode. Transition from vacuum surface discharge to the gas discharge begins at the background pressure of 1-20 Torr, and the surface discharge in this pressure range sustains the gas discharge. At the pressures between 20 and 80 Torr, formation of the discharge stems is completely suppressed, which indicates the range of neutral pressure in the stem of the surface discharge in vacuum. The gas discharge is uniform and it appears on the open surface of the ferroelectric ceramics. In Argon at a distance of 5.5 mm from the surface, the maximal plasma density of /spl sim/10/sup 13/ cm/sup -3/ was observed at pressure of /spl sim/8 Torr. Preliminary study of secondary electron emission from ferroelectrics shows that at low energies of primary electrons the total yield of electron-induced SEE from ferroelectrics is high comparing with other dielectrics and it depends on polarization of the ferroelectric material. Low value of energy which corresponds to the unity yield of SEE from ferroelectrics leads to higher desorption rate and consequently to lower threshold of the surface discharge on ferroelectrics.

Time-dependent transport of both plasma and neutrals is simulated during supersonic molecular beam injection (SMBI) yielding the evolution of edge plasma and neutral profiles. The SMBI model is included as a module, called trans-neut, within the original BOUT++ boundary plasma turbulence code. Results of calculations are reported for the realistic divertor geometry of the HL-2A tokamak. The model can also be used to study the effect of gas puffing. A seven-field fluid model couples plasma density, heat, and momentum transport equations together with neutral density and momentum transport equations for both molecules and atoms. Collisional interactions between molecules, atoms, and plasma include dissociation, ionization, recombination and charge-exchange effects. Sheath boundary conditions and particle recycling are applied at both the wall and the divertor plates. A localized boundary condition of constant molecular flux (product of density times speed) is applied at the outermost flux surface to model the SMBI. Steady state profiles with and without particle recycling are achieved before SMBI. During SMBI, the simulation shows that neutrals can penetrate several centimetres inside the last closed (magnetic) flux surface (LCFS). Along the SMBI path, plasma density increases while plasma temperature decreases. The molecule penetration depth depends on both the SMBI flux and the initial plasma density and temperature along its path. As the local plasma density increases substantially, molecule and atom penetration depths decrease due to their higher dissociation and ionization rates if the electron temperature does not drop too low. Dynamic poloidal spreading of the enhanced plasma density region is observed due to rapid ion flow along the magnetic field (parallel) driven by a parallel pressure asymmetry during SMBI. Profile relaxation in the radial and poloidal directions is simulated after SMBI termination, showing that the plasma returns to pre-SMBI conditions on a time scale of 60 ms.

Plasma opening switches (POS's) have shown excellent characteristics in pulsed power applications. Proposed POS scaling predicts that the fastest opening time for a given conducted current should occur using a high-velocity low-density plasma as the switch medium. The ion beam opening switch (IBOS) uses a charge-neutral ion beam of 100-300 kV, ? 120 A/cm2 as the switch plasma. Its velocity of up to 600 cm/?s and density of ~1012/cm3 make this a very fast low-density plasma compared with typical 10 cm/?s and 1013/cm3 POS plasmas. The IBOS has conducted ? 70 kA flowing in a parallel-plate transmission line driven by a 4-? pulser. IBOS opening time is load dependent, being ? 4 ns into a 15-nH load and about twice as long into a 4-? electron diode load. However, switch impedance is not zero during the entire conduction time, rising to ? 3 ? by the time of peak current. Peak current conducted before opening does not vary linearly with either injected ion current or switch axial length. Instead, the conduction current scales with plasma density in the switch, and is nearly independent of switch area until the area is restricted to a narrow (~1 cm) strip.

We determine conditions allowing for simplification of the description of the impact of a short and arbitrarily intense laser pulse onto a cold plasma at rest. If both the initial plasma density and pulse profile have plane symmetry, then suitable matched upper bounds on the maximum and the relative variations of the initial density, as well as on the intensity and duration of the pulse, ensure a strictly hydrodynamic evolution of the electron fluid without wave-breaking or vacuum-heating during its whole interaction with the pulse, while ions can be regarded as immobile. We use a recently developed fully relativistic plane model whereby the system of the Lorentz–Maxwell and continuity PDEs is reduced into a family of highly nonlinear but decoupled systems of non-autonomous Hamilton equations with one degree of freedom, the light-like coordinate ξ=ct−z instead of time t as an independent variable, and new a priori estimates (eased by use of a Liapunov function) of the solutions in terms of the input data (i.e., the initial density and pulse profile). If the laser spot radius R is finite and is not too small, the same conclusions hold for the part of the plasma close to the axis z→ of cylindrical symmetry. These results may help in drastically simplifying the study of extreme acceleration mechanisms of electrons.

The effects of contaminants on the electrodes of plasma opening switches have been studied. The contaminants introduce backscattered and sputtered neutral atoms into the carbon plasma that is impinging on the cathode. The process has been investigated with the TRIM.SP code. The distribution of density of these neutral atoms from the cathode surface has been obtained and found to be substantial compared to the density of the incident carbon plasma. The dependence of the sputtering yield on the composition of the contaminants has also been studied. >

We present an experimental and simulation-based investigation of the temporal evolution of light emission from a thin, laser-ionized helium plasma source. We demonstrate an analytic model to calculate the approximate scaling of the time-integrated, on-axis light emission with the initial plasma density and temperature, supported by the experiment, which enhances the understanding of plasma light measurement for plasma wakefield accelerator (PWFA) plasma sources. Our model simulates the plasma density and temperature using a split-step Fourier code and a particle-in-cell code. A fluid simulation is then used to model the plasma and neutral density, and the electron temperature as a function of time and position. We then show the numerical results of the space-and-time-resolved light emission and that collisional excitation is the dominant source of light emission. We validate our model by measuring the light emitted by a laser-ionized plasma using a novel statistical method capable of resolving the nanosecond-scale temporal dynamics of the plasma light using a cost-effective camera with microsecond-scale timing jitter. This method is ideal for deployment in the high radiation environment of a particle accelerator that precludes the use of expensive nanosecond-gated cameras. Our results show that our models can effectively simulate the dynamics of a thin, laser-ionized plasma source. In addition, this work provides a detailed understanding of the plasma light measurement, which is one of the few diagnostic signals available for the direct measurement of PWFA plasma sources.

Plasma Opening Switches have been examined numerically with the aid of the ANTHEM plasma simulation model. A generic bi-cylindrical switch is studied. The switching of generator pulses ranging from 50 ns to 1 usec is reviewed, for a variety of plasma fill lengths and densities, and for a range of resistive loads.

The development of a large area (Asource,ITER = 0.9 × 2 m2) hydrogen negative ion (NI) source constitutes a crucial step in construction of the neutral beam injectors of the international fusion reactor ITER. To understand the plasma behaviour in the boundary layer close to the extraction system the 3D PIC MCC code ONIX is exploited. Direct cross checked analysis of the simulation and experimental results from the ITER-relevant BATMAN source testbed with a smaller area (Asource,BATMAN ≈ 0.32 × 0.59 m2) has been conducted for a low perveance beam, but for a full set of plasma parameters available. ONIX has been partially benchmarked by comparison to the results obtained using the commercial particle tracing code for positive ion extraction KOBRA3D. Very good agreement has been found in terms of meniscus position and its shape for simulations of different plasma densities. The influence of the initial plasma composition on the final meniscus structure was then investigated for NIs. As expected from the Child–Langmuir law, the results show that not only does the extraction potential play a crucial role on the meniscus formation, but also the initial plasma density and its electronegativity. For the given parameters, the calculated meniscus locates a few mm downstream of the plasma grid aperture provoking a direct NI extraction. Most of the surface produced NIs do not reach the plasma bulk, but move directly towards the extraction grid guided by the extraction field. Even for artificially increased electronegativity of the bulk plasma the extracted NI current from this region is low. This observation indicates a high relevance of the direct NI extraction. These calculations show that the extracted NI current from the bulk region is low even if a complete ion–ion plasma is assumed, meaning that direct extraction from surface produced ions should be present in order to obtain sufficiently high extracted NI current density. The calculated extracted currents, both ions and electrons, agree rather well with the experiment.

Summary form only given. A double exposure, three-reference-beam, holographic interferometer using a pulsed Nd:YAG laser has been developed by HYTech Research for NRL. This interferometer was used previously in plasma-filled-rod-pinch diode experiments. In this work, we report on the implementation of, and initial results from, this interferometer to study the effects on the implosion dynamics and pinch stagnation phenomena associated with varying the initial gas density profile of an 8-cm, shell-on-shell ("1234"), long-implosion-time (200 to 3 ns), neon gas-puff Z-pinch. The pinch is driven by the Hawk generator with a plasma opening switch that provides an ~0.6-MA pinch current in ~75 ns that remains fairly constant for several hundred ns. Additional diagnostics include K-shell XRDs, current monitors, a vacuum voltmeter, and a 4-channel, XRD zipper array. The initial profile is controlled by varying the ratio of plenum pressures in the two shells. We describe the interferometer as fielded on Hawk, and discuss issues with, and solutions for, obtaining high-quality holographic interferograms. We also compare fast-Fourier-transform and phase-stepping methods for analyzing the fringe patterns. It had been shown that the K-shell yield depends on the initial gas profile. In this work, interferograms of the electron-density distribution in the imploding and stagnating pinch are compared for a range of gas profiles keeping the total mass fixed. The interferograms are correlated with radiation traces and implosion radius calculated from both load inductance and snow-plow modeling

Calculated optimum conditions as to initial electron density and cooling rate of a recombining helium plasma for the production of VUV laser oscillation of the Hell 164 nm line are presented together with an explanation of the population inversion mechanism. For high initial plasma density (~1018 cm-3) rapid cooling (τE ≦10-13 sec) should produce a short pulse laser oscillation. For medium initial density (~ 10-16 cm-3), stationary laser oscillation can be produced by much slower cooling (τE ~ 10-8 sec) but on cooling with τE > 10-7 sec the oscillation can hardly be realized. For lower initial plasma density ( ≦ 1015 cm-3), laser oscillation is not possible because the population inversion obtained is not sufficient.